DESCRIPTION OF THE PRIOR ART
In the manufacture of integrated circuits and similar devices, it is common practice to etch through a first thin layer in a prescribed or predetermined pattern to expose an underlying substrate. When the thin layer is of electrically insulating material, the substrate typically will be either a metal or a semi-conductive material. After the prescribed pattern has been etched through the layer of insulation, another layer of metal or semi-conductive material may be applied over the insulation to come into contact with exposed portions of the underlying substrate. During initial and/or subsequent etching processes, after the outermost layer of material is etched away, an overetching condition begins to take place, the magnitude of which is a function of the plasma etching parameters. If the etching process is not terminated soon after overetching begins, a defective unit may result.
Typically, light reflected from an insulation material undergoing etching will produce a low frequency signal in the order of about 0.005 Hz from an optical detector. This low frequency or undulating signal is produced by constructive and destructive interference of the reflected light waves. When an insulation layer is completely etched away to expose a metal layer, the detector will produce an essentially constant or DC signal to indicate the presence of metal. Prior art systems are known which can detect the transition from insulation to metal but not the transition from metal to insulation. Also, prior art systems of the type incorporating digital signal analysis techniques employ analog to digital converters whose conversion times have no relationship to the process being controlled. Such systems typically require both analog and digital filtering circuitry because of the low frequency signal obtained from the detector.
Various techniques have been developed in the past for monitoring the progress of an etching process to determine when a surface layer has been etched away to reveal an underlying substrate. An etch end-point detector was disclosed by R. N. Price in IBM Technical Disclosure Bulletin Vol. 15, No. 11, pp 3532-33, dated April, 1973, in which light reflected from a surface undergoing etching is monitored to detect changes in the light characteristics indicative of the desired end point. An output signal is generated when the derivative of the light signal with respect to time is zero for a period of time. Thus, etching through an insulation layer to an underlying metal layer may be monitored. Another such end-point detection system was disclosed by J. C. Collins and P. J. Pavone in IBM Technical Disclosure Bulletin Vol. 17, No. 5, pp 1342-43, dated October, 1974; however, this system is not readily adaptable to detect transitions from low derivative to high derivative of the light signal. A related technique also was disclosed by H. Moritz in IBM Technical Disclosure Bulletin Vol. 19, No. 7, pp 2579-80, dated December, 1976, in which the intensity of reflected light is recorded and observed to detect end-point. Finally, R. C. Lewis in U.S. Pat. No. 4,041,404, Apparatus and Method for Detecting When a Measured Variable Represented By a String of Digital Pulses Reaches a Plateau, issued Aug. 9, 1977, disclosed a system for monitoring changes in the derivative of a time-varying signal.
While these prior art end-point detection systems each have certain advantages, none of them provides a closed loop control function which will terminate the etching process when the desired end-point has been reached. Moreover, they appear to be rather sensitive to circuit noise from typical electrical noise sources and also provide no flexibility in analog-to-digital conversion times which would permit the system to track various etching processes having differing characteristics. Finally, their capability to detect the end-point of metal-to-insulation or insulation-to-insulation etching is rather doubtful.